The prior art teaches preparation of trialkyl oxonium salts of HMF.sub.6 wherein M is an element selected from the group consisting of phosphorous, arsenic and antimony (U.S. Pat. No. 3,585,227) that are useful as catalysts for preparation of rubbery polyepihalohydrins (U.S. Pat. No. 3,850,857); and, when water or a glycol is employed as a reactant, liquid hydroxyl-terminated epihalohydrin polymers (U.S. Pat. No. 3,850,856). Co-polymerizations of epichlorohydrin with glycidyl esters of ethylenically unsaturated acids or ethylenically unsaturated epoxides to produce solid elastomers which contain vinyl unsaturation have been disclosed previously in U.S. Pat. Nos. 3,285,870 and 3,158,591. New polymers are desired having an epihalohydrin polymeric backbone but different reactive end groups.